The temporary interconnection of sections of sheet material by stitching or pasting, with formation of intervening seams, is a widely used expedient designed to facilitate the treatment of separate but identical sheets in a continuous manner. Such treatment may include, for example, the shearing of pile of a velvet-type fabric by a blade that has to be raised on the approach of a seam. A pressure roller used for the satinizing of textiles may also have to be lifted across a seam. The detection of such a seam is likewise of importance when the treatment station comprises a cutter designed to separate consecutive sections after the web has undergone a pattern-forming operation.
A system of the general type here considered has been disclosed, for example, in German printed specification No. 1 268 575, published May 22, 1968. There, a plurality of photocells are used to detect a thread impregnated with a fluorescent coloring agent which is incorporated in a fabric being scanned at or in the vicinity of each seam.
Three earlier German patents, all issued in 1955, disclose seam detectors responsive to metallized threads (No. 924,323), to conductive impregnants (No. 925,884) and to a slightly radioactive mass (No. 930,442). German printed specification No. 24 28 112, published Sept. 21, 1978, proposes the use of a sensing head energized at high frequency to detect a metallic insert, such as a strip of tin foil, embedded in the fabric.
Reference may further be made to U.S. patent No. 3,129,484 describing a so-called seam jumper for the control of cloth shears.
Some of these prior-art devices are relatively complex and all of them require a pretreatment of the web in order to facilitate the recognition of a seam. Such a pretreatment, of course, it a time-consuming operation entailing additional costs.
German printed specification No. 1 268 575 particularly discloses four photocells disposed in one row and with equal spacing in order, it is said, to recognize also seams lying at an angle. The photocells control respective relays of which at least two must operate in order to raise a shearing blade with a certain delay depending on the speed of the advancing web.
In the case of overlapping sections, the increased web thickness in the region of overlap could be used to indicate the location of a seam. This would eliminate the need for a special preparation of the web, yet a problem arises from the possibility that a thickness sensor may misinterpret a surface irregularity as an overlap.